In the printing industry it is often desirable or necessary to perform a sequence of operations on individual sheets of paper or paperboard. Certain types of paper stock such as high quality coated papers or corrugated cardboard are not susceptible to running as a roll or web and must be treated as a stack of individual sheets.
In a typical operation, a stack of paper sheets which may be four to five feet high and weigh as much as 5,000 pounds will come from a store room or sheet cutter and will be prepared for printing operations on a jogger/aerator. Once jogged and aerated the stack will then be conveyed to a sheet-feeding printing press, laminator, die cutter, embosser, or other apparatus collectively referred to herein as "converting equipment" or "sheet treating apparatus".
Conventional printing presses and converting equipment are often configured to accept stacks of sheet paper. However, it is important that each sheet in the stack be delivered from one operation to the next without wrinkling, crimping, or soiling. Heretofore paper stacks have been transported on wooden or plastic pallets which provide slots for the entrance of forklift tines beneath the pallet to allow the lifting and positioning of the entire stack without coming into direct contact with individual sheets in the stack.
Although pallet transport of stacks avoids damage to the conveyed paper, it does require that prior to loading a subsequent stack onto a press or converting equipment the previous pallet must be extracted, removed and transported to a location for reloading. Furthermore, pallets are costly and must be periodically replaced. As every load in process throughout a plant must have a pallet beneath it at all times, a significant capital investment in pallets will be present in any extensive printing operation.
In some applications pallets have been eliminated by the substitution of paper slip sheets placed beneath the stack with a gripping edge extending beyond the stack which may be gripped to drag the stack from one position to another. Slip sheets, however, are subject to tearing and must also be removed between each loading of a particular machine.
To reduce friction between a paper stack and the surface over which it is conveyed fixed air cushion tables or air decks have been employed for conveying a stack from between operations within a single apparatus utilizing spring-loaded dogs mounted on chain-driven bars. Portable air cushion tables are also known for transporting loaded slips from one operation to another although these machines have typically required manual dragging of the stack from the portable table to other apparatus.
What is needed is an apparatus for automatically conveying a stack of paper between operations which does not require pallets or slip sheets and yet which preserves the integrity of all sheets within the stack.